José Luiz Fontes Monteiro

Static crystallization of zeolites MCM-22 and MCM-49

Abstract Zeolites MCM-22 and MCM-49, with SiO 2 /Al 2 O 3 molar ratios (SARs) of 30 and 50, were prepared under static conditions using silica, sodium aluminate and hexamethyleneimine. An aging step before the hydrothermal treatment made it possible to obtain good-quality MCM-22 under these reaction conditions. No aging step during gel preparation gave rise to MCM-49. The aging step probably provides the reaction mixture with nuclei that grow in the static hydrothermal crystallization and whose formation conditions are extremely specific. The physico-chemical characteristics of these materials compare well with those for the same zeolites prepared under stirring, except for the morphology of the particles. The MCM-22 samples with SAR=30 are cylindrical particles with a depression in the center, while those with SAR=50 are spheres with a crack along the centerline. MCM-49, with SAR=30, crystallizes as hollow cylinders with approximately the same dimensions as MCM-22 having the same SAR.

Isobutane/2-butene alkylation on zeolite beta: Influence of post-synthesis treatments

The influence of the framework and extraframework composition of zeolite beta on its activity, selectivity, and deactivation behavior during the alkylation of isobutane with 2-butene has been studied by submitting a H-beta sample to different post-synthesis treatments. The parent Hβ sample was prepared by calcination, conventional NH4+ exchange, and further calcination of a commercial TEA-beta. These treatments were seen to produce some framework dealumination with formation of dispersed cationic extraframework aluminum (EFAL) species. The Hβ was then dealuminated by two chemical treatments (hydrochloric acid and ammonium hexafluorosilicate) and by steam calcination, the former procedures leading to almost EFAL-free catalysts. Furthermore, Hβ was also treated with the required amount of ammonium hexafluorosilicate (HFS) in order to remove EFAL species generated during the activation of the parent TEA-beta sample. The catalysts were characterized by XRD, N2 adsorption-desorption isotherms,27Al and29Si MAS-NMR, XPS, and IR spectroscopy with adsorption-desorption of pyridine. In general, dealumination of Hβ leads to less active, selective, and stable catalysts for isobutane/butene alkylation. The chemically dealuminated EFAL-free catalysts were much more active than the steamed sample, which was attributed to the presence of both cationic EFAL compensating framework charge and condensed type EFAL blocking acid sites in the latter catalyst. Furthermore, elimination of EFAL in Hβ by HFS treatment also decreased the alkylation activity of the zeolite, suggesting a synergistic effect of dispersed cationic EFAL species with framework hydroxyls of Hβ to form Bronsted acid sites of enhanced acid strength.

On the stability of MCM-41 after ion-exchange and impregnation with cesium species in basic media

Abstract The stability of MCM-41 after ion exchange and impregnation with cesium species and calcination was investigated. A pure silica and two Al-containing samples (Si/ Al ≅ 25 ), the latter synthesized either in the sodium or in the cesium form, were used as test samples. The conditions chosen for the various treatments were those usually employed to introduce basic species in zeolites or in mesoporous materials aiming at their use in catalysis. The results show that even mild basic media (pH≅8.5) can be detrimental to the stability of the structure and that impregnation with cesium acetate followed by calcination caused a severe damage to the structure. Al-containing samples were less resistant than the purely siliceous one to basic media.

Aldol condensation of acetone over alkali cation exchanged zeolites

The aldol condensation of acetone was studied over faujasite type zeolites with different silica/alumina ratios (X and Y) both in the original Na form and Cs and K exchanged. The pore volume and specific surface of cesium exchanged zeolites were smaller than that of the parent sodium form, since cesium is a more volumous cation. The increase in the zeolite basicity is related to the decrease of the silica/alumina ratio and to the electronegativity of the counter cation. This increase has favoured the mesityl oxide (MO) hydrogenation to methylisobuthylketone (MTBK) and has decreased the selectivity to isobutene (IB).

Influence of Ni content on physico-chemical characteristics of Ni, Mg, Al-Hydrotalcite like compounds

The physico-chemical properties of a series of Ni,Mg,Al-HTLC with Al/(Al+Mg+Ni) = 0.25 and low Ni/Mg ratios were studied by means of X-ray diffraction (XRD), thermogravimetric (TGA) and thermodifferential (DTA) analysis, N2 physissorption and temperature programmed reduction (TPR). The as-synthesized materials were well-crystallized, with XRD patterns typical of the HTLCs in carbonate form. Upon calcination and dehydration the dehydroxilation of the layers with concurrent decomposition of carbonate anions produced mixed oxides with high surface area. XRD analysis indicated that the different nickel and aluminum oxides species are well-dispersed in a poor-crystallized MgO periclase-type phase. As observed by TPR, the different Ni species showed distinct interactions with Mg(Al)O phase, which were influenced by both nickel content and calcination temperature. Regardless of the the nickel content, the reduction of nickel species was not complete as indicated by the presence of metallic dispersions.

The effect of cyclic dealumination of mordenite on its physicochemical and catalytic properties

Abstract Commercial Na mordenite was ion-exchanged with both hydrochloric acid and ammonium chloride. The exchanged samples were subjected to three cycles of hydrothermal/acid treatments, the hydrothermal treatments being performed at 723, 823, and 923 K. Samples having a silica-to-alumina ratio (SAR) in the range 10.7–100 were prepared. They were characterized by XRD, XRF,29Si, and27Al solid-state n.m.r., t.p.d, of NH3, nitrogen physisorption, and XPS N.m.r. results showed that most but not all extraframework alumina (EFAL) could be leached with no damage to the zeolite framework. Considerable mesopore formation in the range 30–50A˚was observed. Also, the EFAL was shown to block micropores, mesopores, and strong acid sites. Steaming promoted migration of EFAL to the external surface, whereas leaching rendered the samples rather homogeneous. The activity form-xylene isomerization displayed a maximum as the aluminum content decreased. Catalytic stability increased with aluminum removal for bothn-heptane cracking andm-xylene isomerization.

Sequential experimental design for parameter estimation: a different approach

Abstract Six different strategies for sequential design for parameter estimation are analysed. Two of them have not been described before. According to the examples studied, the β-trace design criterion, based on the minimization of the trace of the posterior covariance matrix of parameters, seems to be the best. This criterion did not present most of the drawbacks presented by the other five criteria and provided the most accurate parameter estimates.

Aldol Condensation of Citral with Acetone on Basic Solid Catalysts

The catalytic performance of solids with basic properties, such as CaO, MgO and hydrotalcites, was evaluated in the aldol condensation of citral and acetone, the first step in the synthesis of ionones from citral. The best results were obtained with CaO and hydrotalcite with high conversions (98%) and selectivities (close to 70% for the main product) observed for both of the catalyst. Such pseudoionone yields were greater than those reported in the literature for the homogeneous reaction.

Influence of thermal treatments on the basic and catalytic properties of Mg,Al-mixed oxides derived from hydrotalcites

This work studied the influence of calcination conditions on basic properties and catalytic performance of Mg,Al-mixed oxides derived from a hydrotalcite sample (Al/(Al+Mg)=0.20). Various heating rates, calcination atmospheres and lengths of calcination at 723K were evaluated. TPD of CO2 and retroaldolization of diacetone alcohol (DAA) were used to determine the basic properties of the mixed oxides. The basic site density determined by TPD of CO2 showed a better correlation with catalytic activity for acetone/citral aldol condensation than the relative basicity obtained from retroaldolization of DAA. Calcination atmosphere was the parameter that influenced most the basic and the catalytic properties of the Mg,Al-mixed oxides, with calcination under dry air being the best choice.

Characterization of the coke formed from o-xylene over mordenites at different pressures under N2 and H2

Abstract The influence of the carrier gas (H 2 or N 2 ), at different pressures, on coking over two mordenite samples during o -xylene transformation was evaluated at 623 K. The results showed that the carrier gas has a significant effect on the amount and on the composition of the coke formed. For the two samples used, coke formed in the presence of H 2 consisted mainly of CH 2 Cl 2 soluble molecules, while under N 2 the formation of insoluble coke was favoured. Whatever the carrier gas or the total pressure, the same families of compounds were identified in the soluble fraction, the main ones being C n H 2 n −28 , C n H 2 n −22 and C n H 2 n −16 . For the sample strongly dealuminated, increasing the pressure decreased the coke build up under H 2 , confirming the inhibition effect of H 2 on coke formation, but an opposite effect was observed under N 2 . Due to its mesoporous system, this sample has a rather stable activity whatever the carrier gas. For the sample with a high density of acid sites, the limiting quantity of coke was rapidly attained, deactivation was relatively fast and poisoning by coke was greater under N 2 . A relatively small amount of coke render it inactive indicating that deactivation is due to pore mouth blockage.